Insulation of Conductors

. The fourth property of telephone lines, insulation of the conductors, usually is expre ed in ohms as an insulation resistance. In practice, this property needs to be intrinsically high, and usually is measured by millions of ohms resistance from the wire of a line to its mate or to the earth. It is a convenience to employ a large unit. A million ohms, therefore, is called a megohm . In telephone cables, an insulation resistance of 500 megohms per mile at 60° Fahrenheit is the usual specification. So high an insulation resistance in a paper-insulated conductor is only attained by applying the lead sheath to the cable when its core is made practically anhydrous and kept so during the splicing and terminating of the cable. Insulation resistance varies inversely as the length of the conductor. If a piece of cable 528 feet long has an insulation resistance of 6,750 megohms, a mile (ten times as much) of such cable, will have an insulation resistance of 675 megohms, or one-tenth as great.